Black enamel frit obscurations are commonly printed on laminated and tempered automotive safety glazing.
Obscurations have multiple functional and aesthetic requirements.
The obscuration must be substantially opaque. This is needed to prevent the adhesive, used to mount the glazing to the vehicle, from being seen from the outside of the vehicle. It must also protect the adhesive from the harmful effects of ultra-violet. The obscuration also serves this same function for the equipment attached to the inside surface of the glazing. On heated and coated glazing, the obscuration is used to hide the edge of the coating, bus bars, leads and any other items that would detract from the appearance of the vehicle. The obscuration also has the additional aesthetic requirement that it have a dark black color and a reflective glossy appearance.
In addition to hiding the adhesive, equipment mounted to the glazing and other items the obscuration must be durable. It must last for the life of the glazing, without fading, having a reduction in adhesion or otherwise failing. Another important functional requirement is that the obscuration must protect the adhesives from the ultraviolet rays of the sun.
The obscurations are usually printed on the surface 24 of glass facing the inside of the vehicle.
In addition to the obscurations, markings required for regulatory compliance (date, manufacturer, compliance classification, model, etc.) and images, such as trademarks, barcodes etc., are also printed on the glass.
Obscurations have historically been black. One of the reasons for this comes from the limitations imposed by ceramic frits. It is difficult to produce frits, in colors other than black, that have the durability needed and for which the color can be reliably replicated from run to run. The other reason is that a black obscuration can be used with any color of paint.
The practice of applying black enamel frit obscurations became common place the 1980s when the industry switched from the rubber H channel and the butyl adhesive strip windshield mounting systems to polyurethane adhesive mounting of safety glazing. This change was made in response to the poor safety performance of the prior technology. The channel/butyl mounted windshields were often dislodged on impact allowing the occupants to be ejected from the vehicle. To improve occupant retention in crashes, the industry switched to polyurethane adhesive mounting. Today, on new vehicles, butyl is no longer used and the rubber H channel mount is only seen on commercial and off-road vehicles.
One advantage of the butyl strip system was that the strip was narrow enough to be covered by a molding or trim strip to obscure the butyl and the vehicle mounting flange. The rubber channel also did not require an edge obscuration as the rubber covered both the edge of glass and the flange.
This had to change to enable the use of polyurethane. To obtain the required bond strength, between the vehicle and the glass, it was found that the bead of polyurethane had to be substantially wider than the butyl strip had been. With a minimum width of 19 mm, it was no longer practical to obscure the adhesive from view from the outside with a trim strip or molding due to the width that would have been required. In addition, polyurethane needs to be protected from ultra-violet light, UV, to prevent degradation. As a result, the black enamel frit obscuration band was added to the glass, to obscure the view of the polyurethane and to protect the polyurethane by blocking the UV. This obscuration band of black enamel frit that encircles the day light opening is commonly called the “black band”.
While there can be no doubt that the polyurethane system has saved countless lives, it has some disadvantages.
Black enamel frit is comprised of pigments, a carrier, binders and finely ground glass. Other materials are also sometimes added to enhance certain properties: the firing temperate, anti-stick, chemical resistance, etc. The black frit is applied to the glass using a silk screen or ink jet printing process prior to heating and bending. During the bending process, the finely ground glass in the frit soften and fuses with the glass surface. The frit is said to be “fired” when this takes place. This is very similar to the process used to apply enamel finishes on bathroom fixtures, pottery, china and appliances.
Metals and many other types of materials have an ultimate yield strength at which point the material will fail. However, with glass we can only specify a probability of breakage for a given value of stress. Looking at glass at the molecular level, we would expect the strength to be very high. In fact, what we find in practice is that glass has a very high compressive strength, as expected, but very low tensile strength.
For a given set of glass test specimens, with identical loading, the point of failure at first glance might appear to be a random variable. In fact, the yield point follows a Weibull distribution and the probability of breakage can be calculated as a function of, stress, duration, surface area, surface defects and the modulus of glass.
To the naked eye, float glass appears to be near perfect. Any defects that may be present as so small as to not be visible. But, in fact, at the microscopic level, the surface appears rough and can be seen to be dotted with flaws. When the glass is placed in tension, these surface defects tend to open up and expand, eventually leading to failure. Therefore, laminated automotive glass almost always fails in tension. Even when not in tension, the surface defects react with the moisture in the environment and slowly “grow” over time. This phenomenon is known as slow crack growth. As a result, glass weakens as it ages.
A fired black frit increases surface defects. This can be seen if the black pigment of a fired black obscuration is chemically dissolved. The surface of glass will have a frosted appearance, similar to sand blasted or chemically etched glass. The frosted appearance is due to the myriad surface defects present from the fused glass. This makes the surface weaker increasing the probability of breakage. Testing has shown that glass with black frit fails at a stress level that is substantially lower than glass that does not have black frit.
Another problem arises from the thermal gradients that occur during the bending process. As one would expect, the black frit absorbs more radiant heat than the clear glass. Radiant heat is the predominant heat source used for glass bending. The black frit areas of the glass run hotter than the adjacent clear areas. With glass being a poor conductor of heat, gradients in the tens of degrees centigrade can arise over a short distance. These high abrupt thermal gradient on the surface result in optical distortion and high residual along the inner edge of the black band. This is known as the “burn” line in the industry. This can often be seen along the edges of the black obscuration found along the edges of most windshields.
Obscurations used with glass mounted camera systems are forced to designate a “buffer” zone 15 between the edge of the camera field of view 16 and the edge of the black frit 8 (FIG. 9A) to exclude the burn line. This is an accommodation required due to the limitations of the black frit. The ideal would be to have no buffer zone 15 as the larger obscuration reduces the driver field of view and the natural light entering the vehicle (FIG. 9B).
One method used to address the burn line problem is the dot fadeout. Starting at the inner solid edge of the black paint, rows of increasingly smaller dots are painted on the glass. This is the same principle as used in greyscale printing. This reduces the rate of change in the surface temperature, spreading it over a wider area. The dot fadeout also helps to hide the distortion. However, on some parts, even a wide dot fadeout is not sufficient to eliminate all distortion. A wide dot fadeout also may not be possible depending upon the size of the opening and the regulatory requirements for driver vision. Dot fadeout patterns are also undesirable in that they increase the production cost of the glazing.
Another problem is surface mismatch. A laminate is comprised of at least two layers of glass. The frit is typically applied to only one of the glass layers. This can result is a slight difference in the shape of the surfaces. When the two surfaces are forced together during lamination, the mismatch results in residual stress in the laminate and optical distortion.
Even with these drawbacks, the area of the windshield with a black frit obscuration has increased in recent years. There are a number of reasons why this is occurring.
To improve aerodynamics and lower wind noise, the wiper rest position on many vehicles is located out of sight below the hood line. When this is done, the bottom edge of the windshield is extended to form a “wiper rest” 31 so as to provide a resting surface for the wipers when not in use as shown in FIGS. 1, 2 and 3. This area is typically obscured with a black enamel frit 31 extending downward and connecting to the black band. The width of this area can be in excess of 15 cm. This area is also normally heated, to prevent snow and ice from building up, using a screen print conductive silver on the number four 24 surface of the glass. The power density required to keep this area free of ice and snow is double that of a rear window defroster due to the snow and ice that is packed in from the wiper action. The wide wiper rest frit obscuration, with drawbacks intrinsic to frit, coupled with the thermal stress of melting snow and ice, leads to a high breakage rate for these windshields.
As the electronic content of modern vehicles has increased, the area of the windshield, near the top center, has become increasingly crowded on many vehicles. Once the province of just the rear-view mirror, we now find a wide array of equipment mounted in this location.
One of the first devices to compete for this area was the infra-red rain sensor use to provide for a full automatic mode of windshield wiper operation as well as other vehicle functions such as closing the sun roof and pulsing the brakes to keep the rotors dry. The rain sensor must have a lens optically coupled to the number four 24 surface of the windshield. An IR LED shines light onto the number one 21 surface. The amount of light reflected back correlates to the amount of water present on the number one 21 surface. The field of view of the lens must be in the area cleared by the wipers in order to function properly. Therefore, the lens is generally mounted on or near the vertical centerline near the top of the wiped area. Power and signal wiring is required for the sensor to work. In addition to the lens 10, the sensor comprises a housing 12 and a cover 14 all of which need to be hidden from view from the outside of the vehicle to avoid an unsightly clutter appearance.
The rear view mirror itself has undergone a major transformation. Once a purely mechanical device, electro-chromic automatic dimming rear-view mirrors have become a popular option and are standard equipment on many models. The additional components required to provide this functionality make the mirror heavier and therefore require a larger footprint mounting bracket than a standard non-dimming mirror.
The rear-view mirror has further evolved as telematics driver aid systems, integrated garage door openers and hands free blue-tooth interfaces have been introduced. The rear view mirror is a convention place to locate control push-buttons and microphones which require additional wires which also must be protected and covered. Power also needs to be provided to the mirror. A cover is often provided to protect the cable and to hide it from the inside of the vehicle.
The use of cameras, requiring a wide field of view and a high level of optical clarity, is also growing at a rapid rate with the introduction of vehicles capable of various levels of autonomous operation. The resolution of the cameras is also increasing at an equally fast rate. These typically must be mounted on the windshield in the wiper area. Early initial applications were for night-vision. Today, camera based systems are used to provide a wide array of safety functions including adaptive cruise control, obstacle detection, lane departure warning and support for autonomous operation. Many of these applications require the use of multiple cameras. A clear undistorted field of view, with minimal double imaging and excellent MTF (Modulation Transfer Function, a measure of how well a lens maps an image to a sensor), is especially critical for camera based systems to perform as intended. It is essential for these systems to be able to quickly differentiate between objects, capture text, identify signage, and operate with minimal lighting. Further, as the resolution of the cameras used increases the need for a clear distortion free field of view increases.
While covers 14 and various styling methods can be used to obscure the components and cables from the inside, we also need to maintain clean lines and a good aesthetic when the vehicle is viewed from the exterior.
Standard practice has been to extend the black enamel frit band 32 to create an obscuration 8 on the number four 24 surface with openings in the obscuration 30 to provide for the required field of view as shown in FIGS. 1, 2 & 3.
When the black enamel frit band 32 is extended downward from the top center black band to create an obscuration 8 on the number two 22 or number four surface 24, distortion and stress can become a major problem FIG. 1. This is because the black frit is extending further from the edge into the area that where more heat must be applied to bend the glass. The large surface area of the obscuration increases the probability of breakage due to the surface defects and stress introduced by the frit. This is also a critical viewing area.
A panoramic windshield is a windshield in which the top edge of glass has been extended to include at least a portion of the roof giving the driver a vertical field of vision of at least 45 degrees as defined by the applicable regulatory standards.
In the case of a panoramic windshield FIG. 3, the problem is even more pronounced as the obscuration 8 extends several cm from the top edge and is located near the weakest point of the windshield. This is also where the highest temperatures are required to bend the glass.
The process for printing on laminate interlayers was first commercialized in 2003. Some of the key patents include: U.S. Pat. No. 8,002,938 B2, U.S. Pat. No. 7,232,213 B2 and U.S. Pat. No. 7,278,730. To date, the primary application has been for architectural laminates, providing a means to produce images and marking, in vivid colors at a resolution that was never before possible. However, little has been done in the automotive market. DE201110004500 teaches the use of a printed plastic layer for use in an automotive laminate for reducing weight and providing artistic freedom to the designer. US 20080286542 provides for an acoustic laminate with an image printed on film and adhered to the interlayer though use of an adhesion promoter.
One of the reasons why we have not seen the adaptation of printed interlayers in automotive is that the printed interlayers do not meet the functional requirements of an obscuration. A screen print enamel frit provides a high degree of protection from UV which is needed to protect the polyurethane used to bond the glazing to the vehicle opening. Inks printed on the interlayer do not have the same level of opacity and UV blocking ability that the black enamel frits do. They also do not provide the deep black glossy appearance of a fired black frit. This failure is due to the limited types of ink that are available for printing on interlayers.
One of the primary functions of the interlayer is to serve as the adhesive to hold the sheets of glass together. In the event of an accident, the interlayer holds the shards of glass together and helps to prevent the occupant from being ejected. In the event of an impact from the exterior of the vehicle, the interlayer helps to prevent the projectile from penetrating and entering the cabin. Penetration resistance is a key requirement for regulatory compliance. Any ink applied to the interlayer, must not interfere with this essential safety function. The interlayer must be able to bond to the glass through the print. Adhesion promoters are often needed even with the best ink formulations. This requirement severely limits the type of inks that can be used.
Interlayer sheets do not lend themselves well to the automated feed systems of typical printing equipment. The various materials used to produce interlayer sheets are all thermo plastics with glass transition points in the room temperature range or lower. As such, the sheets are limp and difficult to work with. The surface of interlayers for lamination also need to be embossed to facilitate handling and the lamination process. It is difficult to print on an embossed surface.